Bridge gating circuit with floating bias source



June 27, 1961 R, M, MaGINTYRE 2,990,477

BRIDGE GATING CIRCUIT WITH FLOATING BIAS SOURCE Filed Aug. '7, 1956 aCONSTANT CURRENT fz BIAS 1 SOURCE 3 INPUT OUTPUT -r CIRCUIT CIRCUITINPUT SIGNAL l8 OUTPUT SIGNAL United States Patent i f 2,990,477 BRIDGEGATING CIRCUIT WITH BIAS SOURCE "mesne assignments, to Thompson RamoWooldridge Inc.', Cleveland, hio,'a corporation of Ohio Filed Aug. 7,1956, Ser. No. 602,550

4 Claims. (Cl. 307-885) The present invention relates to an electronicgating circuit and, more particularly, to a bridge gating circuit forselectively passing an applied voltage signal.

A conventional bridge gating circuit includes a plural-- ity ofunidirectional current-conductive devices, together with a bias sourcefor supplying a current to the unidirectional devices to bias them inthe conductive region, and

a means for turning the bias current either on or off. When the biascurrent is on, the bridge circuit presentsa lowimpedance and is capableof passing with relatively little attenuation a signal voltage which maybe applied to it. When the bias current is turned off, the applicationof a signal. voltageenconnters a substantially infinite back-impedancepresented by one or more of the unidrrectional current-conductivedevices.

In such a conventional bridge gating circuit, the bias source and themeans for turning the bias current on or oft may be a single circuit, asshown, for example, in U.S. Patent No. 2,577,015, for Switching System,issued December. 4,1951, to W. R. Johnson. Another conventionalarrangement is to provide a bias source and an on-oif control. device astwo separate circuits coupled substantially in parallel to the bridgegating circuit in such a way that the operation of the on-off controldevice is capable of overriding the action of the bias source.Arrangements of this type are described in an article entitled AccurateLinear Bi-Directional Diode Gates, by J. Millman and T. H. Puckett,published in Proceeding of. the IRE, J-anuary 19, 1955, at pages 29-37.

FLOATING f Robert M. Maclntyre, Los Angeles, Califi, assig'n'onby 1 theimpedances The performance of a gating circuit may be indicated by meansof factors known as gain, error voltage output, and transfer function.The gain is a factor less than one which represents the portion of theinput voltage actually transferred to the output circuit. The errorvoltage out 5 put exists because-of the fact that within the gatingcircuit the values of operating voltages or of circuit ele--. mentseither depart from their established design values in the firstinstance, or'else drift fromtheir initial values during'operation of thecircuit. The transferfunction refleets the over-'allperforrnance of thegating circuit including both gain and error vo1-tage output, and isdefined as the ratio of the voltage developed across the output circuitto the voltage supplied bythe input circuit.

It is, of course, desirable that the value of the transfer Q functionshould be maintained at unity. Since the circuit gain is always lessthan unity the transfer function may achieveithe value .ofunity only ifthe error voltage output happens toprovide exactly the rightcompensating effect. -It will be readily appreciated that because ofvari ous considerations it is highly undesirable to rely upon the errorvoltage output for such a purpose. 'Rather, it is desirable to maintainthe value of the transfer function atunity by having the gain be equalto unity andthe.

It is, therefore, an object of the present invention to provide animproved form of electronic gating circuit for selectively passing anappliedsignal voltage in response to, one value of a bi-valued controlvoltage.

Another object ofthe invention is to provide an improved bridge gatingcircuit for selectively passing an applied signal voltage with theminimum amount of attenuation. z. i

Still another object of the invention is toprovide a bridge gatingcircuit for selectively passing an applied sig-" nal voltage, theeffectiveness ofrthe transfer of the signal voltage being substantiallyequal to the ratio of the impedance of an associated output circuit tothe sum of of the output circuit and an associated input circuit.

According to the present invention, it is recognized that" prior artbridge gating'circuits have caused attenuation of a gated signal voltagefor two separate reasons: First, an attenuation resulting from theimpedance presented.

by the unidirectional current-conductive devices of the bridge circuitwhen biased in their conductive state; and,

second, an attenuation resulting froma diversion of a portion of thesignal current through the bias source which.

provides the bias current for, the bridge circuit. The

present invention also recognizes that whereas the first ofthcseattenuation factors appears to be necessary and. unavoidable, the secondfactor can be entirely eliminated, providing a very substantialimprovement in the over-all.

performance of the bridge gating circuit.

Thus, the present invention in its general form provides 1 a bridgegating circuit to which associated signal input and output circuits maybe coupled, a floating bias source for supplying bias current to theunidirectional current-conductive devices of the bridge circuit, and anon-ofi control means for selectively interrupting the flow of biascurrent in order to control the passage of a sig nal voltage from theinput circuit to the outputcircuit. The floating bias source ischaracterized by the fact that I it is electrically isolated from theinput and output'cir Y cui-ts, except for the necessary interconnectionvia-the bridge circuit. Signal current is not drawn into the biaspaththrough source because there is no; longer any return which it mayflow back to the input circuit.

of the present invention than by prior art circuits.

' vides an ingenious manner of mechanizing the'floa'tingconventionalbridge gating" circuits are "presented" and" bias sourceforan application where the gate is to 1 beturned on for periods of lessthan a predetermined length. The bias source provided for an applicationof 11 this type includes a capacitor forstoring a bias voltage,

' 'at least one resistor connected in series with the capacitor toproduce a substantially constant bias current, the ca- 1 pacitor andresistor being coupled across the bridge I gating circuit; and acharging circuit including at :least one unidirectionalcurrent-conductivedevice as a serial; element thereof for replenishingthe charge on the capacitorwhcnthe gating circuit is'in the 011"conditionfeatures of new invention. Which are believed to be novel andpatentable are pointed out in the claims1v which form fa'pait or thisspecification For a, better urideistanding'ofthe invention, reference ismade in PatentedJune 27, 1961* Many advantagesare provided byv thepresent inven- 1 tion as a direct result of the use of a floating biassources Attenuation; of the signal voltage is'minimized, resulting in-asubstantial increase in the, circuit gain, Also," the total errorvoltage output is considerably reduced and the toluances oncircuitcomponentv values are re laxed. vvlurthennore, in some applications, asmaller. number of circuit elements are required by the. circuits thefollowing description to the accompanying drawings, wherein like partsare indicated by like reference numerals in which:

FIGURE 1 is a partial-schematic and partial-block diagram illustratingthe general form of the invention;

FIGURE 2. is a schematic diagram specifically illus hating a' simplifiedversion of the invention; I

FIGURE 3 is a schematic diagram specifically illustrating a preferredform of the constant current bias source of FIGURE 1 and the manner ofits interconnection to the bridge circuit and the on-ofi controlcircuit; and

-FIGURE 4 is a schematic diagram specifically illustrating another formof the constant current bias source. ..Referring now to the drawing,there is illustrated in FIG. 1 an electronic voltage gate comprising a'bridge circuit 10, an on-off control circuit 20, a constant current biassource 30, an input circuit 40 and an output circuit 50. Bridge circuitincludes a pair of bias control terminals 11 and 13, terminal 11 beingbiased positive with respect to terminal 13 When the bridge is toconduct, and being biased negative with respect to terminal 13 when thebridge is notto conduct. A pair of unilaterally conducting devices 15and 16 provide a first conducting branch of the bridge circuit, poled topass current from terminal 11 to terminal 13, and a pair of similardevices 17 and 18 provide a second conducting branch. A pair of signalterminals 12 and 14 are provided at the junction points between devices15, 16, and 17, 18, respectively. Input circuit 40 is connected betweensignal terminal 14 and a lead 55 while output circuit 50 is connectedbetween signal terminal 12 and lead 55.. On-ofi control device 20 iscoupled to the bias control terminals 11, 13, via a unilaterallyconducting device 11, a first output lead of the control circuit beingconnected to the cathode of device 21 whose" anode is connected toterminal 11, a second output lead ofcontrol circuit 20 being connectedto terminal 13. Bias source 30 is connected directly across terminals 11and 13.

.In operation, the four unilaterally conducting devices of bridgecircuit 10 are normally maintained in a con dugtive state by biascurrent flowing, in the conventional sense, out of bias source 30 toterminal 11, thence through the bridge circuit, out of terminal 13 andback to the bias source. Terminal 11 istherefore normally at a potentialwhich is positive with respect to that of terminal 13'. The upper outputleadof on-off'control circuit 20 is normally positive with respect tothe lower output lead, the voltage difference being substantiallygreater than the normal potential difference between terminals .11 and13-, hence device 21 is normally backe biased. Because of the.very lowforward impedance of devices 15-18 a signal voltage generated by' inputcircuit 40 normally encounters a very low impedance between signalterminals 12 and 14, and a very large proportion of the signal voltageappears across outpu circuit 50. g 1

The normal application of signal'voltage from input circuit 40 throughbridge circuit 10 to output circuit 50 may be selectively interrupted bythe operation of the on-otfcontrol circuit 20. When the upper lead ofcon trol circuit 20 assumes a potential which is substantially morenegative than that of its lower output lead; device 21 becomesforward-biased, and hence conductive. Terminal 11 assumes a negativepotent al with respect to terminal 13 and hence all of devices1518-becom'e back-biased and non-conductive, while the constant currentbias source 30 continues to supply a constant" output current which hasnow been diverted from bridge circuit 10 to flow instead through on-oifcontrol circuit 2.0.

Signals generatedby input circuit 40, therefore, do not the bias currenthave a substantially constant value, however, available types ofunidirectional devices are not" vention as illustrated in FIG. 1, itwill be seen that during normal operation when the bridge is conductingthe on-oif control circuit 20 is effectively decoupled by de vice 21.Bias source 30 is' connected to the bridge cirshit but does not providea path for signal current since the potentials of terminals 11 and 13are identical insofar as alternating current is concerned. Therefore,signal current flowing from input circuit 40 to terminal 14 divides atthat point, part of it flowing through devices 17 and 15 to terminal 12,the remainder flowing through devices 18 and 16 to terminal 12, theentire current then flowing into output circuit 50 and thence back toinput circuit 40.

The nature of the floating bias arrangement provided by the inventionmay be conveniently explained with reference to the article by Millm'anand Puckett and, more specifically, to FIG. 10 on page 34 thereof, andto the accompanying discussion which indicates that current flowing fromthe signal input source through the bridge circuit has three possiblereturn paths, a first one through the signal output circuit and secondand third paths through the bias circuits. The present inventioneliminates the second and third return paths for signal current. Thus,in FIG. 1 of the drawing in the present application, it will be notedthat there is an absence 7 of any direct electrical connection betweenthe lead' 55 and bias source 30. v

The significance and operation of the present invention will be moreeasily understood with reference to FIG. 2 in which like parts are giventhe same reference numerals as inFIG. 1. In FIG. 2 the constant currentbias source 30 is illustrated as comprising a battery 31 connected inseries with a resistor 32, where it is assumed that the resistance valueof resistor 32. is very large compared to the impedance values in thebridge circuit. On-off control circuit 20 is illustrated as comprising apair of batteries 23 and 24, connected in oppo site polarities, and aswitch 25 for selectively placing one or the other of the batteries inthe circuit. Input circuit 40 is illustrated as comprising a source ofalter-' nating potential 41 connected in series witha resistor 42, whileoutput circuit 50 is illustrated as a resistor 51.

The operation of the gating circuit of FIG. 2 may be described by theequation:

5, R s s-ir'l' 1.

where E represents the voltage appearing across output RL s L Referenceis again made to the: Millman Bucket t article and, more specificallyto: the" equations forgain G which are set forth in Table l on page 33thereof.

It be readily observed thatthe gain equationfor the circuit ofthe'present invention is much. siniplerand contains far fewer factors.It may also be easily estab-:.

lished-that, for the same set of circuit values in the inputand outputcircuits and the gating circuit, the present invention provides asubstantially higher value of gain than do any of the conventional ingcircuits. 1

It will be shown that the present invention, while maintaining all ofthe advantages of prior art bridge gating I circuits, and Whileproviding a higher value of gain, also eliminates entirely an importantsource of error voltage output. In the article by Millman and Puckett,

types'of bridge gata factor U(E) (see Table 1) indicates the fraction ofthe unbalance in bias voltages which appears across the output circuit.The unbalance in bias voltages is equal to the numerical differencebetween the supply voltages plus B and minus E, illustrated in FIG. 9and FIG..'10-

of the Millman and Puckett article. it is important to note that where ahigh degree of ac- In this connection, v

curacy and of precision is required, the difficulty of pro- FIG. 3 doesnot show input circuit 40 or'outputcircuit 50, but does include agrounded lead or reference tercondiillustrating a pre-.

minal 55, and indicates that the input signal maybe applied betweenterminals 14 and 55, while the output signal may' be derived betweenterminals 12 and 55. The constant current bias source is not connectedto ground terminal 55, but is electrically isolated from the input andoutput circuits except for the intercoupling via bridge circuit 10, thusproviding the floating bias source of the present invention.

In FIG. 3 a flip-flop 26 is illustrated as a preferred type of on-oifcontrol circuit, coupled to the bridge circuit via a pair ofunilaterally conducting devices 21, 22. As is well known in the art, aflip-flop may be conveniently utilized to provide a pair of steady-stateelectrically controlled signals whose values are complementary, that is,one signal is high when the other signal is low, and vice-versa. It willbe understood th-at'a source of energy, not shown, may be coupled to thefiip-. flop, and that some form of input circuit, not shown, may, beutilized for selectively changing the flip-flop from one to the other ofits stable states. The flip-flop may, if desired, be connected at somesuitable point to ground terminal 55, since it is not essential to thepresent invention that the input and output circuits be isolated fromthe onoff control device. The reason for this is that the on-ofi controldevice is isolated from the bridge circuit by the back-biasing ofdevices 21 and 22 when the bridge circuit is conductive.

The preferred form of constant current bias source 30 as illustrated inFIG. 3 includes a resistor 33 connected to bias control terminal 11 ofthe bridge circuit, a resistor 34 connected to the other end of resistor33, an energy storage device such as a capacitor 35 connected toresistor 34, and a resistor 36 connected between capacitor 35 andterminal 13. Capacitor 35 has its upper plate norm-ally chargedpositively with respect to its lower plate and provides bias current tobridge circuit 10 through a series loop which includes resistors 33, 34,and 36. A unilaterally conducting device 61 has its anode connected tothe lower output lead of flip-flop 26 and its cathode connected to thejunction between resistors 33 and 34, while a unilaterally conductingdevice 62 has its cathode connected to the upper output lead offlip-flop 26 and its anode con. nected to the junction between capacitor35 and resistor 36. Devices 61 and 62 are normally back-biased when thebridge circuit 10 is conductive. However, when flipflop 2 6' isselec'tively operated so that itsupperi output lead! becomesnegativewith respect to its lower output:lead, 1:

the "bridge circuitis biased E-non;conductive, then current flows fromflip-flop 26 through device 61,i"resistor=. 34, capacitor 35', anddevice 62,*and hence back to:fiip'=: flop 26, thus replenish-ingthecharge on capacitor 35; The purpose ofresistor 34is to limitthecharging' current to capacitor 35.: During the-times when the bridgecircuit-x. is non-conductive and the flip-flop 'is supplying energy :11through devices 61*and' 62 to replenish the chargeo'n: capacitor 35, theconstant-current bias source 30 never-'. theless'continues to supply asubstantially constant cur-:0-

rent to terminals 11 and 1'3.- Thus again, as in the circuit of FIG. 2,the constant current bias source is not turned 01f but merely has itsoutput current by-p'assed througha' different path when the bridgecircuit is non-conductive:

Since it is generally preferred not topermit the :value" of the biascurrent to vary substantially, thepefiodof time during which the-gate ispermitted to conduct must belimitedwith due regardto the valuev ofcapacitor-35:: and to the.otftime available for replenishing the chargeIn an application in which the circuit of FIG. 3 was actually used, thegatewas operated inan on the'capacitor'.

regular cyclic-fashion, being turned on for approximately 150microseconds, then off for approximately l50rnicro-' 5 seconds," and soon.

Although. other circuit values may be used, a satisfactory'set of valueswhich have been successfully employed in the circuit ofFIG. 3 are listedas follows:

All unilaterally conductingtdevices are siliconldiodest Resistors 33, 36ohms ,1,0 ,000 Resistor 34 V do 1,000 Capacitor 35 microfarads 10Flip-flop 26 is able to supply 3 milliamperes ofcurrent 10 volts betweenits two output at a voltage difference of terminals.

73, a transformer secondary 74, a'unilaterally conducting devicej 1having its cathode connected to the junction between resistor 71 andcapacitor 72 and its anode connected to one end ofsecondary winding 74,and a resistor 76 connectedbetween theother end of secondary winding 174 and the other terminal of capacitor 72. .Resistor 7.6

serves to limitthe charging current to. capacitor;-72.. In operation,the application of energy in alternating form across transformer primary73 serves to maintain the upper plate of capacitor 72 at a positivepotential with respect to its lower plate, thus continuously supplyingcurrent to the bridge circuit.

The advantages provided by the present invention may now be convenientlysummarized in view of the foregoing description of the various circuitmodifications illustrated in the drawing. The floating bias source ofthe present invention produces a substantial improvement in the overalloperation of the gate,

Not only is the value of the circuit gain substantially nearer to unity,but the total error voltage output is greatly reduced, while at the sametime, limitations on the values of the various circuit parameters arerelaxed. For a particular application in which the gating circuit is tobe turned on for time periods of less than a predetermined length, thepresent invention has also provided a very useful and ingenious circuitwherein the bias current may be supplied from a capacitor when the gateis on, and the charge on the capacitor may be replenished when the gateis off.

It is, of course, to be understood that the present invention is notlimited to the use of semi-conductor diodes as the unilaterallyconducting devices, but extends as well to any other type ofunilaterally conducting devices' which are now available, or' may becomeso in the future. Also} the advantage of the floating bias source is notnecessarily limited to a bridge circuit utilizing four unilaterallyconducting devices, but may be applicable as well to a bridge circuit inwhich only two of the arms contain such devices. Moreover, although theembodi ment' of FIG. 3 shows the input andoutput signals as having acommon, grounded terminaLiWith the'bias. cir-' cuit as'ungrounded, itshould'be clear that thebias circuit could be grounded if the commonterminal were not, it only being necessary that the bias source not bedirectly coupled to the input and output circuits. Although theconducting state of the bridge circuit has been referred to in thedescription as being the normal state, it is to be understood that thisis merely a matter of definition, and that the opposite definition couldhave been used with the bridge" circuit being normally non-conductingand being made to conduct by selective operation of the on-ofi controlcircuit. Many other forms and circuit modifications, in addition to'those specifically shown and described hereing fall within the scope ofthe invention and will be readily apparent to those skilled in the art.

What I claim as new and desire to secure by Letters Patent'of the UnitedStates is:

1. An electronic gating circuit comprising, in combination: a bridgecircuit including first, second, third, and fourth diodes, the cathodeof said first diode being connected to the anode of said second diode,the cathode ofsaid third diode being connected to the anode of saidfourth diode, the anodes of said first and third diodes being connectedtogether, the cathodes of said second and fourth diodes beingconnectedtogether; a constant current source coupled between the'junctionor said'firs't and third diodes'and the junction of said secondand fourth diodes; a two-terminal input circuit, one terminal thereofbeing connected to the junction of said third and fourth diodes; atwo-terminal output circuit, one terminal thereof being connected to(the junction of 'said first and second diodes; means to connect theother terminal of said output circuit to the other terminals of saidinput circuit only; and a current conducting path including switch meansoperative to selectively direct the current from said constant currentsource through said bridge circuit when a signal from said input circuitis to be passed to said output circuit and for directing the currentfrom said constant current source through said current conducting pathwhen a signal from said input circuit is to be inhibited from passing tosaid output circuit.

2. A current source selectively operable for supplying a substantiallyconstant current to an output circuit for second output-terminals; a"first resistor, a capacitoryand a second resistor coupled seriallytogether in the" order named between said first and second outputterminals; an on oif control circuit having first and second outputleads, Isaid first output lead being normally biased'negatively withrespect to said second output lead; first and second unilaterallyconductive devices, each having an anode and a cathode, said firstdevice having its anode connected to said second output lead and its'cathode con nected to the junction between said first resistor and saidcapacitor, said second device having its anode connected to the junctionbetween said capacitor and said second resistor and its cathodeconnected to said first output lead, whereby said capacitor normallyacquires a charge having a predetermined polarity; a third unilaterallyconductive device connected to one of the output leads of said on-offcontrol circuit, providing a series circuit connected across said firstand second output terminals so that current normally flows from saidfirst output terminal to said first output lead, and from said secondoutput lead to said second output terminal; and means coupled to saidon-off control circuit far selectively biasing said first output leadpositively with respect to said second output lead. during said timeperiod.

3. The gating circuit of claim 1, wherein said constant current sourceincludes a capacitor; and means for charging the capacitor of saidconstant current source when the signal from said input circuit is beinginhibited from passing to said output circuit.

4. The gating circuit of claim 3, wherein said constant current sourcefurther includes at least one resistor connected in series with thecapacitor; and said means for charging the capacitor of said constantcurrent source includes means for connecting the capacitor to the switchmeans of said current conducting path during the charge interval.

References Cited in the file of this patent UNITED STATES PATENTS2,098,370 Bartels Nov. 9, 1937 2,505,688 Picking Apr. 25, 1950 2,563,406Goldberg Aug. 7, 1951 2,625,662 Gaynor et a1 Jan. 13, 1953 2,782,307 VonSivers et al Feb. 19, 1957 2,817,757 Durbin Dec. 24, 1957 2,866,103Blake et a1 Dec. 23, 1958 UNITED STATES PATENT OFFICE CERTIFICATE OFCORRECTION Patent No, 2 99O 47'I June .27 1961 Robert M MacIntyre It ishereby certified that error appears in the above numbered pat-- entrequiring correction and that the said Letters Patent should readies"corrected below.

Column 2 line 5O for 'toluances" read tolerances column 8, line 25 f "fam 0 read for Signed and sealed this 1st dayyof May 1962.,

(SEAL) I Attest:

ERNEST SWIDER DAVID LADD -Attesting Officer Commissioner of Patents

